Estimation and analysis of compaction‐induced traveltime shifts: Methodology and parametric study

Abstract

Modeling of time shifts associated with time-lapse (4D) seismic surveys is helpful in evaluating reservoir depressurization and inverting for subsurface stress. Here, we discuss time shifts estimated from synthetic seismic data and analyze their dependence on reflector depth and pressure drop inside the reservoir. Accurate time shift measurements between baseline and monitor surveys are obtained by advanced processing techniques that are potentially applicable to field data. Coupled geomechanical and seismic modeling is used to study time shifts for parameters close to those for West Texas petroleum reservoirs. Time-shift leads and lags for P-, PS-, and S-waves are estimated for a wide range (10% to 50%) of effective depressurizations. For the largest pressure drop, P-wave time lags reach 45 ms for reflectors above the reservoir, while S-wave leads reach 90 ms for reflectors below the reservoir. We also investigate the contributions of the deviatoric and volumetric strains to the time shifts, and show that the deviatoric strain is largely responsible for P-wave anisotropy near the reservoir. Time shifts for S-waves and, to a lesser extent, PSwaves, are strongly influenced by the volumetric strain inside the reservoir. Moderate tilt of a rectangular reservoir, or its replacement with an elliptically shaped reservoir of the same aspect ratio, has little influence on time shifts. Potentially, our methodology can be applied to invert for compaction-induced stress fields using simple compartmentalized reservoir models.